1. Field of the Invention
The invention relates generally to optical imaging and in particular to systems and methods for achieving flexibility in interference fringe visibility control and optimization of signal to noise ratio, as well as for achieving polarization insensitivity, dispersion matching and optical output polarization control in optical coherence domain reflectometry (OCDR) or optical coherence tomography (OCT).
2. Description of Related Art
Optical coherence domain reflectometry (OCDR) is a technique initially developed to provide a higher resolution over optical time domain reflectometry (OTDR) for the characterization of the position and the magnitude of reflection sites in such optical assemblies as optical fiber based systems, miniature optical components and integrated optics (Youngquist et al., “Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique”, 1987, Optics Letters 12(3):158–160). With the addition of transverse scanning, this technique has been widely and successfully extended to the imaging of biological tissues, and is termed optical coherence tomography (OCT) (Huang, D., E. A. Swanson, et al. (1991). “Optical coherence tomography.” Science 254 1178–81; and U.S. Pat. Nos. 5,321,501 and 5,459,570). Since then, a large number of applications have been found for this technology as evidenced by a number of review articles (Swanson E. A. et al. “Optical coherence tomography, Principles, instrumentation, and biological applications” Biomedical Optical Instrumentation and Laser-Assisted Biotechnology, A. M. Verga Scheggi et al. (eds.) pages: 291–303, 1996 Kluwer Academic Publishers, Printed in the Netherlands; Schmitt, J. M. “Optical coherence tomography (OCT): a review” IEEE Journal of Selected Topics in Quantum Electronics, Volume: 5, Issue: 4, Year: July/August 1999, pages: 1205–1215; Fujimoto, J. G. et al. “Optical Coherence Tomography: An Emerging Technology for Biomedical Imaging and Optical Biopsy” Neoplasia (2000) 2, 9–25; Rollins A. M. et al. “Emerging Clinical Applications of Optical Coherence Tomography” Optics and Photonics News, Volume 13, Issue 4, 36–41, April 2002; Fujimoto, J. G. “Optical coherence tomography for ultrahigh resolution in vivo imaging.” Nat Biotechnol 21(11): 1361–7, (2003)). Each of these documents is incorporated herein by reference.
The most straightforward and most commonly used interferometer configuration for OCDR or OCT is a standard Michelson interferometer. As shown in FIG. 1, light from a low coherence source 110 is input into a beam splitter or 2×2 fiber optic coupler 112, where the light is split and directed into a sample arm 114 and a reference arm 116. An optical fiber 118 in the sample arm 114 extends into a device 120 that scans an object 122. The reference arm 116 provides a variable optical delay. Light input into the reference arm 116 is reflected back by a reference mirror 124. A piezoelectric modulator 126 may be included in the reference arm 116 with a fixed reference mirror 124, or the modulator 126 may be eliminated by scanning the mirror 124 in the Z-direction. The reflected reference beam from reference arm 116 and the scattered sample beam from sample arm 114 pass back through the coupler 112 to detector 128 (including processing electronics), which processes the signals by techniques that are known in the art to produce a backscatter profile or image on a display unit 130.
This configuration is advantageous in that it uses a minimum number of optical components and is hence the simplest. It can be implemented using bulk or fiber optics or a combination thereof. However, this configuration is limited to an optical efficiency of 25% as explained below.
By examining the configuration, it is not difficult to discover that the optical power reaching the detector from the two arms is reciprocal with respect to the beam splitter or fiber coupler (BS/FC). Assuming that the power split ratio of the beam splitter is
  α      1    -    α  and neglecting loss in the splitter, the attenuation by the beam splitter or the fiber coupler (BS/FC) to both the sample optical wave and the reference optical wave is the same and is equal to α(1−α), the only difference is that for one wave it will propagate straight-through the BS/FC first with an attenuation by a factor of α and then crossover the BS/FC with a further attenuation by a factor of (1−α), whereas for the other wave, it will crossover the BS/FC first with an attenuation by a factor of (1−α) and then propagate straight-through the BS/FC with a further attenuation by a factor of α. It is well known to those skilled in the art that for such a configuration, the most efficient power splitting ratio is 50/50, where
            α              1        -        α              =    1    ,simply because the function α(1−α) has its maximum value when α=0.5. For example, with a 50/50 power split ratio, for either the sample arm or the reference arm, the optical power is firstly attenuated at the BS/FC by 50% from the light source to the sample or reference arm and then further attenuated by 50% from the sample or reference arm to the detector, which leads to a total overall power attenuation factor of 50%×50%=25% for both arms. If the BS/FC power split ratio is 90/10, then for the reference and the sample arm respectively, the total overall power attenuation factor by the BS/FC will be 90%×10% and 10%×90%, which is the same and is equal to only 9%.
Various configurations have been proposed to improve the optical power efficiency. The configuration described in this patent is simpler than those previously proposed designs and also addresses polarization fading issues that are not addressed by the other high optical efficiency designs.
Rollins and Izatt (U.S. Pat. No. 6,657,727; Andrew M. Rollins, Joseph A. Izatt “Optimal interferometer designs for optical coherence tomography” Optics Letters, Vol. 24, Issue 21, Page 1484 (1999)) proposed a number of interferometer configurations to improve the optical efficiency of the above Michelson interferometer configuration. As shown in FIG. 2, a key optical element that is used in all their configurations is a commercially available non-reciprocal device called an optical circulator and such a circulator is combined with unbalanced couplers, and (or) balanced heterodyne detection for optical power efficient interferometer construction. In contrast, the design we describe herein eliminates the optical circulator, a complex and expensive component. Our design is also very compact and relatively low cost as it uses a minimum number of optical elements.
It should be pointed out that FIG. 2 encompasses six configurations, where the three insets (FIGS. 2Aii; 2Bii and 2Cii) basically show a modification from the three corresponding balanced heterodyne detection approach employing balanced couplers to a single detector based detection employing unbalanced coupler(s) as shown in the main FIGS. 2Ai; 2Bi and 2Ci. Refer now to the first two configurations (FIGS. 2Ai and 2Aii), which are based on a Mach-Zehnder interferometer with the sample 222 located in a sample arm 214 and the reference optical delay line (ODL) 225 in the reference arm 216. In the case of 2Ai, the main difference from a standard Mach-Zehnder interferometer is that the prior fiber coupler 212 has an optical power split ratio of
      α    1        1    -          α      1      instead of 50/50 that is optimized for optical power efficient high SNR detection by directing most of the original optical power to the sample arm 214 and meanwhile light is coupled to the sample 222 through an optical circulator 232 such that the backscattered optical signal is collected by the delivery fiber 218 but is redirected to the post fiber coupler 234. The reference arm ODL 225 may be transmissive using, for example, a fiber wrapped PZT based fiber stretcher or it may be retroreflective using, for example, a corner mirror or cube combined with another optical circulator (not shown, see U.S. Pat. No. 6,657,727). Note that in FIG. 2Ai, the post fiber coupler 234 has a split ratio of 50/50 and due to the employment of balanced heterodyne detection 236, Izatt and Rollin showed that the SNR of FIG. 2Ai can be improved over that of a standard Michelson configuration as shown in FIG. 1.
In the configuration of FIG. 2Aii, the post fiber coupler 238 is also made non-50/50 and a single detector 240 is used. The advantage of FIG. 2Aii embodiment as compared to FIG. 2Ai embodiment is that since only one detector is used, the cost of the system will be lower than that of FIG. 2Ai.
Refer now to FIGS. 2Bi and 2Bii, while the sample arm part is the same as in FIGS. 2Ai and 2Aii, the reference arm ODL 242 is made retroreflective but without the need of a second optical circulator. Again, the optical power split ratio of both the prior fiber coupler 244 and the post fiber coupler 246,
                    α        1                    1        -                  α          1                      ⁢                  ⁢    and    ⁢                  ⁢                  α        2                    1        -                  α          2                      ,can be properly chosen for either the two detector based balanced heterodyne detection case 248 or the unbalanced single detector case 250 to optimize the SNR such that the system is optical power efficient. Izatt and Rollin showed that the SNR improvement of the FIGS. 2Bi and 2Bii embodiment is very similar to that of FIGS. 2Ai and 2Aii embodiments. Note that there will be a small portion of the optical power from the reference ODL 242 being returned to the light source path.
The configurations of FIGS. 2Ci and 2Cii are basically Michelson interferometer based and their difference as compared to FIG. 1 is the use of an optical circulator 252 in between the light source 254 and the fiber coupler 256 to channel the returned light from the fiber coupler 256 initially propagating towards the light source 254 now completely to the detector, d2. While for balanced heterodyne detection, the optical power split ratio of the fiber coupler 256 must be made 50/50, it should be noted that for the case of a single detector unbalanced detection 258 (FIG. 2Cii), the optical power delivered to detector d2 from the sample arm 260 and the reference arm 262 can be made different or non-reciprocal since for detector d2, the sample optical signal will propagate straight-through the fiber coupler 256 twice and the reference optical signal will cross-over the fiber coupler 256 twice. As a result, the optical power delivery to detector d2 can be made efficient by properly selecting the fiber coupler optical power split ratio
      α          1      -      α        .Izatt and Rollin stated that for the configuration shown in FIGS. 2Ci, the SNR can be improved over that of FIG. 1 and although this configuration is not as power efficient as the other two, i.e. FIGS. 2Ai and 2Bi, its significant advantage is that it can be easily retrofitted with a circulator in the source arm and with a balanced receiver, with no need to disturb the rest of the system. As for FIG. 2Cii, the SNR improvement is similar to that of FIGS. 2Aii and 2Bii.
As an extension to all their configurations, Izatt and Rollin included, in their patent (U.S. Pat. No. 6,657,727), three more configurations as shown in FIG. 3 in which a transmissive sample is in the place of the circulator and the sample. They defined a transmissive sample as any sample illumination and collection geometry in which the illumination and collection optics occupy separate optical paths. Such designs have significant alignment issues and are not relevant to the invention being described where the illumination and collection optics occupy the same optical path.
As can be seen from the above-mentioned various configurations, the key advantage of these prior configurations lies in the improvement of the optical power delivery efficiency to the detector(s), by properly selecting an optical power split ratio
  α      1    -    α  (for either the prior and/or the post fiber coupler).
Another issue with the classic Michelson interferometer (FIG. 1) is that light from the reference arm is coupled back into the optical source, causing side effects that can impact the quality of the resulting image. Most of the configurations proposed by Izatt and Rollins address this issue as does the invention described herein. An issue not addressed by Izatt and Rollins configurations above is polarization fading, or loss of signal associated with mismatches between the polarization states of the light from the reference and sample arms. These mismatches are caused by birefringence and its fluctuations in the sample and reference arms, generally dominated by the birefringence in the optical fibers.
For a retraced light wave, placement of Faraday rotators at the ends of the fibers has been shown in the prior art to eliminate polarization fading due to the fiber optic components. FIG. 4 shows the approach of using two Faraday rotator mirrors at the end of the two arms of a standard Michelson fiber optic interferometer to eliminate polarization fading (Kersey, A. D. et al. “Polarization-insensitive fiber optic Michelson interferometer”, Electronics Letters, Volume: 27, Issue: 6, pages: 518–520, (1991)). In this design, the Faraday rotator and mirror enable birefringence compensation in a retraced fiber path for both the sample arm and the reference arm. Although this design solved the problem of polarization fading, it did not address the issue of optical efficiency as the optical splitter configuration is the same as the standard Michelson interferometer configuration of FIG. 1. The invention described herein takes advantage of the polarization rotation caused by the Faraday rotators to increase the optical efficiency of the system by introducing a polarizing beam splitter in the source arm for coupling the light returning toward the source into a detector. This leads to an unbalanced optical efficiency assuming no birefringence in the sample and the use of a polarized source. An additional advantage of such a system is that the light being collected on the detector is linearly polarized, which is advantageous for spectral domain optical coherence tomography and reflectometry systems.
In spectral domain OCT systems, the light is dispersed by a diffraction grating and collected by an array of detectors. The efficiency of the diffraction grating is generally polarization dependent, and thus can be made most efficient for linearly polarized light. As will be elaborated later, the present invention can meet such a requirement.
In order to partially address the polarization fading problem, U.S. Pat. No. 6,564,089 by Izatt et al. mentioned the provision of a polarization compensation means such as a Faraday rotator on the side of the light emission of the optical probe on top of some of the interferometer configurations as discussed before with respect to FIG. 2. By doing so, the OCT can obtain a stabilized interference output regardless of the state of the bend of the sample arm. The inclusion of a Faraday rotator at the end of the sample arm optical probe only is particularly related to the application of OCT to endoscopic biological imaging in which the sample arm optical probe beam needs to be rotated to acquire cross sectional images of a tubular tissue and hence the birefringence property of the sample arm is very vulnerable to fluctuations. A drawback of such a system is the additional cost of the Faraday rotator and furthermore, while polarization compensation is provided for the sample arm, the same is not provided for the reference arm and as a result, there will be a mismatch in the birefringence as well as the dispersion properties between the two arms. Obviously, any birefringence fluctuation in the reference arm will still cause polarization fading and at the same time, the final output optical polarization of the configuration is not predetermined and hence is not suitable for spectral domain OCT which is polarization dependent.
In terms of addressing the polarization fading issue, besides using Faraday rotators, an alternative approach is to use polarization-maintaining (PM) fibers. In addition, a so-called polarization diversity receiver (PDR) scheme (Sorin, et al. “Polarization independent optical coherence-domain reflectometry” U.S. Pat. No. 5,202,745) can also be used. There are also combinations in which PM-fiber, polarization control optical elements and FRM are used (Everett M. et al. “Birefringence insensitive optical coherence domain reflectometry system” U.S. Pat. No. 6,385,358). PM fibers have several issues associated with their two orthogonal polarization axes, which make them undesirable for commercial OCDR or OCT applications. These include variable optical dispersion, difficulties in maintaining high polarization extinction in the connection between two PM-fibers or between a PM-fiber and a polarization optical component, and high cost.
FIG. 5 shows Sorin, et al.'s polarization independent optical coherence-domain reflectometry configuration (U.S. Pat. No. 5,202,745), where the light returning from the sample and reference arms is split into two orthogonal polarization modes with each mode being detected by a separate detector. In this design, a linear polarizer in the reference arm is adjusted to compensate for birefringence in the reference arm so as to equal signal powers on each detector in the detector arm in the absence of a signal from the test, or sample, arm. The problem with this approach is that the polarizer needs to be adjusted as the birefringence in the reference arm changes. As the birefringence in the non-PM reference arm fiber is strongly affected by temperature and stress, the system must be recalibrated with each use, and suffers from polarization drift during use.
An alternate design for a fiber optic polarization insensitive OCDR system with non-PM fiber in the sample arm has previously been described (Kobayashi et al, “Polarization-Independent Interferometric Optical-Time-Domain Reflectometer”, 1991, J. Lightwave Tech. 9(5):623–628). The reference arm in this system consists of all PM optical fiber. As the two arms use different types of optical fibers, their dispersion properties are drastically different, which hence will lead to loss of resolution due to mismatched dispersion between the sample and reference arms. The system also requires a specialized 50/50 coupler.
U.S. Pat. No. 6,385,358 disclosed a hybrid system involving the use of PM fibers, non-PM fibers and Faraday rotators. An important feature in this patent is the use of a 22.5° Faraday rotator in the beam path to enable a double path rotation of the polarized beam returned from reference arm so that the beam is equally split into two orthogonal polarization modes to interfere with the two corresponding but not necessarily equally split components of the beam from the sample arm, which are then detected by two detectors. By summing the interference signal envelops from the two detectors, the final signal is made independent of the birefringence of the sample arm in a similar way as in the case of a polarization diversity receiver. In addition to polarization insensitivity, the dispersion property of the sample arm is also matched with that of the reference arm to eliminate the dispersion effects that degrade image resolution. Furthermore, arbitrary power split ration α/(1−α) fiber coupler is also used to enable high efficiency optical power delivery to the detector. Considering that for medical applications, the portion of the fiber optic interacting with the patient must be changed for hygienic reasons, a non-PM fiber is incorporated into the sample arm to accommodate a disposable section at the end of the sample arm that interacts with the sample. However, a major disadvantage of the disclosed designs is that the system configuration is not simple at all, as it involves length matched PM fiber and non-PM fiber between the sample and references arms, their splices or connections and the use of a relatively large number of various optical components such as (PM or non-PM) fiber coupler, free space polarization beam splitter (PBS), various Faraday rotators of different rotation angles, and two photodetectors. In the case of a 22.5° Faraday rotator which is placed between a single PM fiber and a single mode non-PM fiber, the light beam needs to be expanded from a first fiber, collimated, passed through the Faraday rotator, and then refocused into the other fiber. All of these make the system both quite complicated and also expensive.
Given the problems with the systems described above, there is obviously a need to combine the benefit of optical power delivery efficiency with polarization insensitivity as well as dispersion matching in a simply configuration that will lower the cost and enhance the performance. The present invention addresses the above-mentioned problems and significantly improves on the prior art systems by effectively achieving high optical power delivery efficiency, polarization insensitivity and also dispersion matching, in a more compact, more robust, and also less expensive manner.